T7 rna polymerase variants with cysteine-serine substitutions

ABSTRACT

The present disclosure provide novel variants of T7 RNA polymerase. Embodiments of T7 variants, according to the instant invention, include a Cysteine-Serine substitution on position 723 of the amino acid sequence of the T7 polypeptide. Embodiments of T7 variants according to the instant invention have a DNA-dependent RNA polymerase enzymatic activity and a reduced tendency to form intramolecular homodimers by way of oxidizing thiol groups. The amino acid substitutions within the T7 variants disclosed herein impact minimally, if at all, the RNA polymerase activity of the T7 polypeptide. Further, the mutations of the disclosed embodiments may optionally be combined with mutations which provide enhanced thermostability compared to the wild-type reference.

PRIORITY CLAIM

This application claims the benefit of European Patent Application No11160799.0, filed Apr. 1, 2011, the disclosure of which is herebyincorporated by reference in its entirety.

SEQUENCE LISTING

The instant application contains a Sequence Listing which has beensubmitted in ASCII format via EFS-Web and is hereby incorporated byreference in its entirety. Said ASCII copy, created on Mar. 28, 2012, isnamed SEQUENCE_LISTING_(—)27368US.txt, and is 425,986 bytes in size.

FIELD OF THE DISCLOSURE

The present disclosure pertains generally to the fields of molecularbiology and protein biochemistry. More particularly, the instantdisclosure pertains to the field of enzyme engineering.

BACKGROUND OF THE DISCLOSURE

T7 RNA polymerase (E.C. 2.7.7.6.), also referred to herein as “T7polymerase” or “T7,” is a monomeric bacteriophage encoded DNA directedRNA polymerase which catalyzes the formation of RNA in the 5′→3′direction. In the process of initiation of transcription, T7 recognizesa specific promoter sequence, the T7 promoter. T7 consists of 883 aminoacids and has a molecular weight of 99 kDa. At the amino acid sequencelevel, T7 is highly homologous to T3 RNA polymerase and, to a lesserextent, SP6 RNA polymerase. The three-dimensional structure of T7 isalso similar to other polymerases with different template and substratespecificities, despite low sequence similarity. T7 consists of differentdomains, the N-terminal domain, the “thumb”, the “palm” and the“fingers” (as described in Sousa, R., and Mukherjee, S., Prog. Nucl.Acid Res. Mol. Biol. 73 (2003) 1-41).

Cloning and expression of the gene encoding T7 has been described inliterature (see, U.S. Pat. No. 4,952,496, the disclosure of which isexpressly incorporated by reference). Conformational changes of T7during transcription (see, Ma, K., et. al., Proc. Nat. Acad. Sci. 102(2005) 17612-17617), the facilitation of promoter clearance of T7 (see,Guillerez, J., et al., Proc. Natl. Acad. Sci. 102 (2005) 5958-5963) andthe abortive cycling phenomenon of T7 (see, He, B., et al., J. Mol.Biol. 265 (1997) 275-288) have all been studied.

T7 has been classified as having high promoter specificity and RNApolymerase enzymatic activity, and as being useful for a variety ofapplications in molecular biology. In the field of recombinant proteinexpression, T7 is used for the high-level expression of recombinantgenes in E. coli (for example, as discussed in Studier, F. W., andMoffat, B. A., J. Mol. Biol. 189 (1986) 113-130). The synthesis ofdefined oligoribonucleotides was described by Milligan, J. F., et al.,Nucl. Aids Res. 15 (1987) 8783-8798. However, for at least someapplications with which T7 RNA Polymerase may be utilized a more stableT7 RNA polymerase would be of value.

SUMMARY OF THE DISCLOSURE

According to the instant invention, novel variants of T7 polymerase areprovided. According to embodiments of the invention, a T7 variant isprovided which includes a Cysteine-Serine substitution on position 723of the amino acid sequence of the T7 polypeptide. According to someembodiments, a T7 variant has a DNA-dependent RNA polymerase enzymaticactivity and a reduced tendency to form intermolecular homodimers by wayof oxidizing thiol groups is provided. The amino acid substitutions,according to the instant invention, have none (or at most only minimal)impact on the RNA polymerase activity of the T7 polypeptide. Accordingto the instant disclosure, the mutations disclosed herein can optionallybe combined with even further mutations, including mutations whichprovide enhanced thermostability compared to the wild-type reference.

According to embodiments of the present disclosure, variants of T7 RNApolymerase with Cys-Ser substitution mutations leading to improvedstability are provided. Further, the instant disclosure providescombinations of several T7 RNA polymerase mutations in a single T7variant (double-, triple-, quadruple-, multiple-mutant), with theproviso that the combined mutations lead to an even increased stability,that is to say enhanced storage characteristics. Even further, thepresent invention provides mutations which give rise to T7 variantswhich additionally exhibit enhanced stability in thermal unfoldingassays.

The present invention also provides improved variants of T7 RNApolymerase by introducing mutations which reduce one or more thiol(=mercapto, =—SH, =sulfhydryl) group(s) of the enzyme while retainingenzymatic activity. For example, some embodiments of the instantdisclosure include amino acid substitutions with Serine at any of thepositions Cys125, Cys347, Cys492, Cys515, Cys723, Cys839, andcombinations thereof.

According to some embodiments of the instant disclosure, an aqueoussolution lacking a reducing agent with a thiol group is provided. Theaqueous solution comprises a variant polypeptide of T7 RNA polymerasewhich has DNA-dependent RNA polymerase activity and a different aminoacid sequence from SEQ ID NO.:2. The variant also includes a Cysteineresidue at amino acid position between 510 and 530, the numbering beingfrom the N-terminus of SEQ ID NO.:2. Further, the variant includes aSerine residue substitution for the Cysteine residue at amino acidposition 723, again numbered from the N-terminus of SEQ ID NO.:2. Evenfurther, in the aqueous solution the variant lacks homomultimerformation of intermolecular disulfide bonds.

According to some such embodiments, the variant may also include one ormore of the Cysteine-Serine substitutions selected from the groupconsisting of Cys125Ser, Cys347Ser, Cys492Ser, Cys515Ser, and Cys839Ser.According to other such embodiments, the variant may also include one ormore of the amino acid substitutions selected from the group consistingof Val426Leu, Val426Ile, Val426Phe, Ser633Val, Ser633Met, Val650Leu,Thr654Leu, Ala702Val, and Val795Ile.

Another embodiment of the instant disclosure includes a method ofsynthesizing a RNA molecule. Embodiments of this method include the stepof providing an aqueous solution devoid of a reducing agent with a thiolgroup. The solution has a variant polypeptide of T7 RNA polymerase. Thevariant also has DNA-dependent RNA polymerase activity and an amino acidsequence different from SEQ ID NO:2. Further, the variant is devoid ofhomomultimer formation of intermolecular disulfide bonds when in thesolution. Embodiments of this method also include the step of providinga template DNA molecule comprising a T7 promoter functionally linked toa target nucleotide sequence to be transcribed. Embodiments of thismethod also include the steps of contacting, within the solution, thetemplate DNA molecule with the variant in the presence of ribonucleosidetriphosphates, and incubating the solution, following the step ofcontacting, under conditions favoring RNA polymerase activity, therebysynthesizing the RNA molecule.

According to some such embodiments, the variant may also include one ormore of the Cysteine-Serine substitutions selected from the groupconsisting of Cys125Ser, Cys347Ser, Cys492Ser, Cys515Ser, and Cys839Ser.According to other such embodiments, the variant may also include one ormore of the amino acid substitutions selected from the group consistingof Val426Leu, Val426Ile, Val426Phe, Ser633Val, Ser633Met, Val650Leu,Thr654Leu, Ala702Val, and Val795Ile.

According to yet other embodiments of the instant disclosure, a methodof producing an aqueous solution devoid of a reducing agent including athiol group is provided. According to such embodiments, the solutioncomprises a variant of T7 RNA polymerase having DNA-dependent RNApolymerase activity and an amino acid sequence different from SEQ IDNO:2. The variant is devoid of homomultimer formation of intermoleculardisulfide bonds when in the solution. According to such embodiments, themethod comprises the step of providing the variant by substituting aCysteine residue at amino acid position 723, the positions numbered fromthe N-terminus of SEQ ID NO.:2, with a Serine residue. Such embodimentsalso include the steps of reverse-transcribing the amino acid sequenceof the variant, thereby obtaining a nucleotide sequence encoding thevariant. Such embodiments also include the step of expressing a nucleicacid molecule comprising the nucleotide sequence of the variant obtainedin the step of reverse transcribing in an expression system, therebyexpressing a polypeptide, and purifying the polypeptide expressed in thestep of expressing, by way of chromatography using an aqueous mobilephase devoid of a reducing agent including a thiol group, therebyobtaining the solution with the variant.

According to some such embodiments, the variant may also include one ormore of the Cysteine-Serine substitutions selected from the groupconsisting of Cys125Ser, Cys347Ser, Cys492Ser, Cys515Ser, and Cys839Ser.According to other such embodiments, the variant may also include one ormore of the amino acid substitutions selected from the group consistingof Val426Leu, Val426Ile, Val426Phe, Ser633Val, Ser633Met, Val650Leu,Thr654Leu, Ala702Val, and Val795Ile.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of this disclosure, and the manner of attaining them, willbecome more apparent and the disclosure itself will be better understoodby reference to the following description of embodiments of thedisclosure taken in conjunction with the accompanying drawing.

FIG. 1 is a stained SDS polyacrylamide gel (following electrophoresis)having wild-type T7 RNA polymerase in both lanes 1 and 2, the T7 RNApolymerase of lane 2 was treated with reducing reagents (DTT).

FIG. 2 is a stained SDS polyacrylamide gel (following electrophoresis)having: wild-type T7 RNA polymerase without His-tag ((Lane 1); wild-typeT7 RNA polymerase including His6-tag (Lane 2); T7 variant #8 [Cys125Ser,Cys347Ser, Cys492Ser, Cys515Ser, Cys723Ser, Cys839Ser] includingHis6-tag (Lane 3); T7 variant #6 [Cys723Ser] including His6-tag (Lane4); T7 variant #3 [Cys347Ser] including His 6-tag (Lane 5); T7 variant#7 [Cys839Ser] including His6-tag (Lane 6); T7 variant #2 [Cys125Ser]including His6-tag (Lane 7); T7 variant #4 [Cys492Ser] includingHis6-tag (Lane 8); and T7 variant #5 [Cys515Ser] including His6-tag(Lane 9), in which no reducing agent was added prior to electrophoresis.

FIG. 3 is a stained SDS polyacrylamide gel (following electrophoresis)having: wild-type T7 RNA polymerase without His-tag ((Lane 1); wild-typeT7 RNA polymerase including His6-tag (Lane 2); T7 variant #8 [Cys125Ser,Cys347Ser, Cys492Ser, Cys515Ser, Cys723Ser, Cys839Ser] includingHis6-tag (Lane 3); T7 variant #6 [Cys723Ser] including His6-tag (Lane4); T7 variant #3 [Cys347Ser] including His 6-tag (Lane 5); T7 variant#7 [Cys839Ser] including His6-tag (Lane 6); T7 variant #2 [Cys125Ser]including His6-tag (Lane 7); T7 variant #4 [Cys492Ser] includingHis6-tag (Lane 8); and T7 variant #5 [Cys515Ser] including His6-tag(Lane 9), in which 10 mM DTT was added prior to electrophoresis toprovide reducing conditions.

Corresponding reference characters indicate corresponding partsthroughout the several views. Although the drawings representembodiments of the present disclosure, the drawings are not necessarilyto scale and certain features may be exaggerated in order to betterillustrate and explain the present disclosure. The exemplifications setout herein illustrate an exemplary embodiment of the disclosure, in oneform, and such exemplifications are not to be construed as limiting thescope of the disclosure in any manner.

BRIEF DESCRIPTION OF THE SEQUENCE LISTING

SEQ ID NO.: 1 is the DNA sequence encoding wild-type T7 DNA-dependentRNA polymerase, including start codon encoding N-terminal methionine;corresponding to #1 in Table 3.

SEQ ID NO.: 2 is the wild-type T7 DNA-dependent RNA polymerase, aminoacid sequence including N-terminal methionine; corresponding to #1 inTable 3.

SEQ ID NO.: 3 is the DNA sequence encoding the Cys125Ser variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #2 in Table 3.

SEQ ID NO.: 4 is the sequence of Cys125Ser variant of T7 DNA-dependentRNA polymerase, amino acid sequence including N-terminal methionine;corresponding to #2 in Table 3.

SEQ ID NO.: 5 is the DNA sequence encoding the Cys347Ser variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #3 in Table 3.

SEQ ID NO.: 6 is the Cys347Ser variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #3 in Table 3.

SEQ ID NO.: 7 is the DNA sequence encoding the Cys492Ser variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #4 in Table 3.

SEQ ID NO.: 8 is Cys492Ser variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to #4in Table 3.

SEQ ID NO.: 9 is the DNA sequence encoding the Cys515Ser variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #5 in Table 3.

SEQ ID NO.: 10 is Cys515Ser variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to #5in Table 3.

SEQ ID NO.: 11 is the DNA sequence encoding the Cys723Ser variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #6 in Table 3.

SEQ ID NO.: 12 is the Cys723Ser variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #6 in Table 3.

SEQ ID NO.: 13 is the DNA sequence encoding the Cys839Ser variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #7 in Table 3.

SEQ ID NO.: 14 is Cys839Ser variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to #7in Table 3.

SEQ ID NO.: 15 is the DNA sequence encoding the Cys125Ser, Cys347Ser,Cys492Ser, Cys515Ser, Cys723Ser, Cys839Ser variant of T7 DNA-dependentRNA polymerase, including start codon encoding N-terminal methionine;corresponding to #8 in Table 3.

SEQ ID NO.: 16 is Cys125Ser, Cys347Ser, Cys492Ser, Cys515Ser, Cys723Ser,Cys839Ser variant of T7 DNA-dependent RNA polymerase, amino acidsequence including N-terminal methionine; corresponding to #8 in Table3.

SEQ ID NO.: 17 is the DNA sequence encoding the Val426Leu variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #9 in Table 3.

SEQ ID NO.: 18 is Val426Leu variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to #9in Table 3.

SEQ ID NO.: 19 is the DNA sequence encoding the Cys723Ser, Val426Leuvariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #10 in Table 3.

SEQ ID NO.: 20 is Cys723Ser, Val426Leu variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #10 in Table 3.

SEQ ID NO.: 21 is the DNA sequence encoding the Val426Ile variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #11 in Table 3.

SEQ ID NO.: 22 is Val426Ile variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to#11 in Table 3.

SEQ ID NO.: 23 is the DNA sequence encoding the Cys723Ser, Val426Ilevariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #12 in Table 3.

SEQ ID NO.: 24 is Cys723Ser, Val426Ile variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #12 in Table 3.

SEQ ID NO.: 25 is the DNA sequence encoding the Val426Phe variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #13 in Table 3.

SEQ ID NO.: 26 is Val426Phe variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to#13 in Table 3.

SEQ ID NO.: 27 is the DNA sequence encoding the Cys723Ser, Val426Phevariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #14 in Table 3.

SEQ ID NO.: 28 is Cys723Ser, Val426Phe variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #14 in Table 3.

SEQ ID NO.: 29 is the DNA sequence encoding the Ser633Met variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #15 in Table 3.

SEQ ID NO.: 30 is Ser633Met variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to#15 in Table 3.

SEQ ID NO.: 31 is the DNA sequence encoding the Cys723Ser, Ser633Metvariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #16 in Table 3.

SEQ ID NO.: 32 is the DNA sequence encoding the Cys723Ser, Ser633Metvariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #16 in Table 3.

SEQ ID NO.: 33 is the DNA sequence encoding the Val650Leu variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #17 in Table 3.

SEQ ID NO.: 34 is Val650Leu variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to#17 in Table 3.

SEQ ID NO.: 35 is the DNA sequence encoding the Cys723Ser, Val650Leuvariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #18 in Table 3.

SEQ ID NO.: 36 is Cys723Ser, Val650Leu variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #18 in Table 3.

SEQ ID NO.: 37 is the DNA sequence encoding the Thr654Leu variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #19 in Table 3.

SEQ ID NO.: 38 is Thr654Leu variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to#19 in Table 3.

SEQ ID NO.: 39 is the DNA sequence encoding the Cys723Ser, Thr654Leuvariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #20 in Table 3.

SEQ ID NO.: 40 is Cys723Ser, Thr654Leu variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #20 in Table 3.

SEQ ID NO.: 41 is the DNA sequence encoding the Ala702Val variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #21 in Table 3.

SEQ ID NO.: 42 is Ala702Val variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to#21 in Table 3.

SEQ ID NO.: 43 is the DNA sequence encoding the Cys723Ser, Ala702Valvariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #22 in Table 3.

SEQ ID NO.: 44 is Cys723Ser, Ala702Val variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #22 in Table 3.

SEQ ID NO.: 45 is the DNA sequence encoding the Val795Ile variant of T7DNA-dependent RNA polymerase, including start codon encoding N-terminalmethionine; corresponding to #23 in Table 3.

SEQ ID NO.: 46 is Val795Ile variant of T7 DNA-dependent RNA polymerase,amino acid sequence including N-terminal methionine; corresponding to#23 in Table 3.

SEQ ID NO.: 47 is the DNA sequence encoding the Cys723Ser, Val795Ilevariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #24 in Table 3.

SEQ ID NO.: 48 is Cys723Ser, Val795Ile variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #24 in Table 3.

SEQ ID NO.: 49 is the DNA sequence encoding the Ala702Val, Val795Ilevariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #25 in Table 3.

SEQ ID NO.: 50 is Ala702Val, Val795Ile variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #25 in Table 3.

SEQ ID NO.: 51 is the DNA sequence encoding the Cys723Ser, Ala702Val,Val795Ile variant of T7 DNA-dependent RNA polymerase, including startcodon encoding N-terminal methionine; corresponding to #26 in Table 3.

SEQ ID NO.: 52 is Cys723Ser, Ala702Val, Val795Ile variant of T7DNA-dependent RNA polymerase, amino acid sequence including N-terminalmethionine; corresponding to #26 in Table 3.

SEQ ID NO.: 53 is the DNA sequence encoding the Val426Leu, Ala702Valvariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #27 in Table 3.

SEQ ID NO.: 54 is Val426Leu, Ala702Val variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #27 in Table 3.

SEQ ID NO.: 55 is the DNA sequence encoding the Cys723Ser, Val426Leu,Ala702Val variant of T7 DNA-dependent RNA polymerase, including startcodon encoding N-terminal methionine; corresponding to #28 in Table 3.

SEQ ID NO.: 56 is Cys723Ser, Val426Leu, Ala702Val variant of T7DNA-dependent RNA polymerase, amino acid sequence including N-terminalmethionine; corresponding to #28 in Table 3.

SEQ ID NO.: 57 is the DNA sequence encoding the Val426Leu, Val795Ilevariant of T7 DNA-dependent RNA polymerase, including start codonencoding N-terminal methionine; corresponding to #29 in Table 3.

SEQ ID NO.: 58 is Val426Leu, Val795Ile variant of T7 DNA-dependent RNApolymerase, amino acid sequence including N-terminal methionine;corresponding to #29 in Table 3.

SEQ ID NO.: 59 is the DNA sequence encoding the Cys723Ser, Val426Leu,Val795Ile variant of T7 DNA-dependent RNA polymerase, including startcodon encoding N-terminal methionine; corresponding to #30 in Table 3.

SEQ ID NO.: 60 is the DNA sequence encoding the Cys723Ser, Val426Leu,Val795Ile variant of T7 DNA-dependent RNA polymerase, including startcodon encoding N-terminal methionine; corresponding to #30 in Table 3.

SEQ ID NO.: 61 is the DNA sequence encoding the Val426Leu, Ala702Val,Val795Ile variant of T7 DNA-dependent RNA polymerase, including startcodon encoding N-terminal methionine; corresponding to #31 in Table 3.

SEQ ID NO.: 62 is Val426Leu, Ala702Val, Val795Ile variant of T7DNA-dependent RNA polymerase, amino acid sequence including N-terminalmethionine; corresponding to #31 in Table 3.

SEQ ID NO.: 63 is the DNA sequence encoding the Cys723Ser, Val426Leu,Ala702Val, Val795Ile variant of T7 DNA-dependent RNA polymerase,including start codon encoding N-terminal methionine; corresponding to#32 in Table 3.

SEQ ID NO.: 64 is Cys723Ser, Val426Leu, Ala702Val, Val795Ile variant ofT7 DNA-dependent RNA polymerase, amino acid sequence includingN-terminal methionine; corresponding to #32 in Table 3.

SEQ ID NO.: 65 is the DNA sequence encoding the Cys125Ser, Cys347Ser,Cys492Ser, Cys515Ser, Cys723Ser, Cys839Ser, Val426Leu, Ala702Val,Val795Ile variant of T7 DNA-dependent RNA polymerase, including startcodon encoding N-terminal methionine; corresponding to #33 in Table 3.

SEQ ID NO.: 66 is Cys125Ser, Cys347Ser, Cys492Ser, Cys515Ser, Cys723Ser,Cys839Ser, Val426Leu, Ala702Val, Val795Ile variant of T7 DNA-dependentRNA polymerase, amino acid sequence including N-terminal methionine;corresponding to #33 in Table 3.

SEQ ID NO.: 67 is the DNA sequence encoding the Val426Leu, Val650Leu,Ala702Val, Val795Ile variant of T7 DNA-dependent RNA polymerase,including start codon encoding N-terminal methionine; corresponding to#34 in Table 3.

SEQ ID NO.: 68 is Val426Leu, Val650Leu, Ala702Val, Val795Ile variant ofT7 DNA-dependent RNA polymerase, amino acid sequence includingN-terminal methionine; corresponding to #34 in Table 3.

SEQ ID NO.: 69 is the DNA sequence encoding the Cys723Ser, Val426Leu,Val650Leu, Ala702Val, Val795Ile variant of T7 DNA-dependent RNApolymerase, including start codon encoding N-terminal methionine;corresponding to #35 in Table 3.

SEQ ID NO.: 70 is Cys723Ser, Val426Leu, Val650Leu, Ala702Val, Val795Ilevariant of T7 DNA-dependent RNA polymerase, amino acid sequenceincluding N-terminal methionine; corresponding to #35 in Table 3 SEQ IDNO.: 71 is the DNA sequence encoding the C125S, C347S, C492S, C515S,C723S, C839S, V426L, V650L, A702V, V795I variant of T7 DNA-dependent RNApolymerase, including start codon encoding N-terminal methionine;corresponding to #36 in Table 3.

SEQ ID NO.: 72 is C125S, C347S, C492S, C515S, C723S, C839S, V426L,V650L, A702V, V795I variant of T7 DNA-dependent RNA polymerase, aminoacid sequence including N-terminal methionine; corresponding to #36 inTable 3.

SEQ ID NO.: 73 is the DNA encoding N-terminal Histidine (His6) tag withlinker sequence, fused to the first two N-terminal amino acids of T7(Met and Asn).

SEQ ID NO.: 74 is the Amino acid sequence of N-terminal Histidine (His6)tag with linker sequence, fused to the first two N-terminal amino acidsof T7 (Met and Asn).

SEQ ID NO.: 75 is the Histidine tag (amino acids); this region mayencompass 3 to 7 “His” residues.

SEQ ID NO.: 76 is the His6 tag (amino acids).

Although the sequence listing represents an embodiment of the presentdisclosure, the sequence listing is not to be construed as limiting thescope of the disclosure in any manner and may be modified in any manneras consistent with the instant disclosure and as set forth herein.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE DISCLOSURE

The embodiments disclosed herein are not intended to be exhaustive orlimit the disclosure to the precise form disclosed in the followingdetailed description. Rather, the embodiments are chosen and describedso that others skilled in the art may utilize their teachings.

The instant disclosure provides the surprising and unexpected discoverythat the T7 RNA polymerase Cys-Ser mutations disclosed herein do notimpact equally on intramolecular disulfide bond formation of T7polymerase. Further, in embodiments of the instant disclosure it isdisclosed herein that, surprisingly, Cys723 is an important residue withregards to homomultimer formation. Further, in some embodiments Cys839may be involved in forming disulfide bridges in homomultimers. Forexample, according to some embodiments of the instant disclosure, a T7variant with only a Cys723Ser substitution, homomultimer formation isdetectably absent. According to an embodiment of the instant disclosure,a T7 variant with only a C839S substitution homomultimer formation issubstantially reduced. Very surprisingly, according to anotherembodiment of the instant disclosure, a sextuple amino acid substitutionT7 variant [C125S, C347S, C492S, C515S, C723S, C839S] isindistinguishable from a T7 variant with only a C723S substitution, asfar as homomultimer formation is concerned. Also surprising, thesextuple T7 variant disclosed herein and the T7 variants with only aC723S substitution or only a C839S substitution exhibit RNA polymeraseactivity similar to that of the wild-type T7 enzyme. As disclosedherein, amino acid substitution mutations on the positions C125, C347,C492, C515, C723, and C839 are very useful and advantageous forconstructing variant T7 polymerase enzymes with reduced tendency to formmultimers with disulfide bridges.

The formation of disulfide bonds can occur during or after the synthesisof certain proteins. Intramolecular disulfide bonds can significantlycontribute to the stability of proteins. Disulfide bond formation alsooccurs between different protein molecules. Cysteine residues involvedin this process have to be located at the surface of the proteinmolecule and have to be solvent exposed. Additionally, the cysteineresidues have to be in a suitable geometry and distance to each other tofacilitate the formation of a disulfide bond.

By way of forming one or more intermolecular disulfide bond(s) betweenproteins, di-, tri-, tetra-, penta-, and higher degree of multimericprotein complexes can be generated. However, uncontrolled andunfavourable formation of such protein complexes by disulfide bondsunder artificial conditions (e.g. in purified preparations of proteins)can impact on biological (e.g. enzymatic) activity or functionality.Therefore, efforts are made to keep proteins under reducing conditions.A common method is the addition of reducing agents to the storagebuffers; examples for such reducing agents are mercaptoethanol,dithiothreitol (DTT), dithioerythritol (DTE) and others. However, thiolgroup reducing agents have a limited shelf life as they themselves areoxidized by molecular oxygen. As a consequence, thiol reducing agentsare losing their protective activity over time, and oxidation ofcysteine residues together with disulfide bond formation can again takeplace.

If a protein sample contains a mixture of different polypeptides or ifthe sample is a crude extract, formation of multimers is possiblebetween molecules of different protein species (heteromultimers). If themixture contains several purified polypeptides of defined activities,formation of heteromultimers between these proteins can negativelyinfluence such activities, affecting single polypeptides separately orall members of the sample.

According to the instant disclosure, T7 RNA polymerase (such as thewild-type T7 RNA polymerase) contains 12 cysteines each having freethiol as functional group. When the thiol groups of two cysteineresidues are brought near each other, an oxidation reaction can generatea cystine unit with a disulfide bond (—S—S—). Storage of T7 RNApolymerase in the absence of thiol reducing agents results in reducedenzymatic activity. Reduction of activity is correlated with theformation of dimers, trimers and even multimers of the enzyme. Thekinetics of disulfide bond formation are strongly dependent onparameters like protein concentration, reaction time, temperature andthe presence or absence of oxidizing reagents. Therefore, in practice,the percentage of multimers present in a sample can vary depending onthe origin, age and composition of the sample. Even during thepurification procedure intermolecular disulfide bond formation can beobserved sometimes.

T7 multimers such as dimers, trimers or even higher degree multimers,may exhibit decreased polymerase activity, possibly due to a reducedaccessibility of the active sites. A reduced flexibility of theindividual polypeptide backbones could also negatively impact enzymaticfunction. Although polymerase activity can be restored by adding freshlyprepared —SH reagents, it is required to continuously monitor residualactivity of the enzyme. In order to safeguard and/or regain enzymaticactivity and/or provide an enzyme preparation with defined T7 polymeraseactivity, it is further necessary to frequently manipulate the material,for example by removing and analyzing samples and reacting T7preparations with reducing agents, etc.

DTNB [5,5′-dithiobis-(2-nitrobenzoic acid), also known as Ellman'sreagent] is a chemical which can be used to quantify the number orconcentration of thiol groups in a polypeptide. A thiol group reactswith DTNB, cleaving the disulfide bond to give 2-nitro-5-thiobenzoate(NTB⁻), which ionizes to the NTB²⁻ dianion in water at neutral andalkaline pH. The NTB²⁻ ion has a yellow color and can be determined, forexample, spectrophotometrically. The reaction of thiol groups and DTNBis rapid and stoichiometric, with one mole of thiol releasing one moleof NTB (for example, the reactivity of T7 RNA polymerase with DTNB hasbeen previously determined as disclosed in Mukherjee, S., et al., Cell110 (2002) 81-91). Per one native polypeptide of wild-type T7 polymeraseenzyme the equivalent of about 2 reactive Cys residues were found butfor the denatured wild-type T7 enzyme the number of reactive Cysresidues determined was 12.

According to the instant disclosure, in at least some applications withwhich T7 RNA Polymerase may be utilized, such as in vitro transcriptionand in vitro amplification methods including Nucleic Acid Sequence BasedAmplification (“NASBA”), Transcription mediated amplification (“TMA”),and other related methods and applications, increasing the stability ofthe T7 RNA polymerase would be beneficial. For example, it would bebeneficial to have a T7 RNA polymerase with an increased storage timewherein any loss of polymerase enzymatic activity is minimized and/or aT7 RNA polymerase which can be stored at room temperature with onlyminor or no loss of enzymatic activity. Also, combining the technicalfeatures leading to enhanced storage characteristics with other featuresleading to enhanced thermostability (e.g., higher reaction temperaturesof isothermal amplification) could allow the amplification of RNA havingsecondary structures. It has also been shown with the polymerase chainreaction (PCR) technology that high annealing temperatures allow thespecific hybridization of a primer to its target resulting in a highlyspecific amplification. With the same advantage, more thermostableenzymes could in principle also be applied to isothermal amplifications.

According to some embodiments of the instant disclosure, a T7 RNApolymerase variant having improved stability, for example enhancedstorage characteristics and a reduced tendency to form multimers withdisulfide bridges, is provided. According to some embodiments, the T7RNA polymerase variants provided herein combine the enhanced stabilityfeatures with polymerase activity at reaction temperatures higher thanthose of the wild-type T7 enzyme.

According to the present disclosure, Cys723 is a cysteine residue of T7polymerase which contributes to disulfide bond formation. For example,according to the instant disclosure, Cys723Ser variants, even after longterm storage of such purified variants, analysis by way of SDS gelelectrophoresis reveals only a single band under non-reducingconditions. This indicates that the homodimer formation is completelysuppressed by the introduction of this single specific mutation. Noother Cys-Ser substitution was found to produce a similar effect in a T7variant with RNA polymerase activity, i.e. in an active enzyme.

As also disclosed herein, the combination of six mutations within a T7gene results in an active enzyme. The six-fold mutant (Cys125Ser,Cys347Ser, Cys492Ser, Cys515Ser, Cys723Ser, Cys839Ser) also shows asingle band under non-reducing conditions (and reducing conditions,too). This indicates that dimer formation can be suppressed completelyby the introduction of Cys-Ser substitution mutations according to theinvention.

The present invention provides novel variants of T7 polymerase, variantswhich are characterized by a different composition of amino acidscompared to the wild-type T7 RNA polymerase polypeptide (wild-typereference). Such a “variant” is an allelic form of the wild-type T7protein, wherein the T7 variant is generated by way of amino acidsubstitution. A T7 variant according to the invention is furthercharacterized by a DNA-dependent RNA polymerase enzymatic activity, anda reduced tendency to form intramolecular dimers by way of oxidizingthiol groups. The identified amino acid substitutions according to theinvention appear to impact only minimally, if at all, on the biologicalfunction of the T7 polymerase polypeptide, including RNA polymeraseactivity. The mutations can optionally be combined with even furthermutations, including mutations which provide enhanced thermostabilitycompared to the wild-type reference.

Certain terms are used with particular meaning or are defined for thefirst time in this description of the present invention. For thepurposes of the invention, the terms used are defined by theirart-accepted definitions, when such exist, except that when thosedefinitions conflict or partially conflict with the definitions setforth below. In the event of a conflict in definition, the meaning of aterm is first defined by any of the definitions set forth in thisdocument.

The term “comprising” is used in the description of the invention and inthe claims to mean “including, but not necessarily limited to”.

The articles “a” and “an” are used herein to refer to one or to morethan one (i.e. to “one or more” or “at least one”) of the grammaticalobject of the article. By way of example, “an amino acid” means oneamino acid or more than one amino acid.

If not stated otherwise, it is understood that the term “about” incombination with a numerical value n (“about n”) indicates a value x inthe interval given by the numerical value ±5% of the value, i.e.n−0.05*n≦x≦n+0.05*n. In case the term “about” in combination with anumerical value n describes a preferred embodiment of the invention, thevalue of n is most preferred, if not indicated otherwise.

A nucleotide sequence “encodes” a peptide or polypeptide when at least aportion of the nucleic acid, or its complement, can be directlytranslated to provide the amino acid sequence of the peptide or protein,or when the isolated nucleic acid can be used, alone or as part of anexpression vector, to express the peptide or protein in vitro, in aprokaryotic host cell, or in a eukaryotic host cell.

Where a nucleotide sequence is single-stranded, it is to be understoodthat the complementary sequence of that nucleotide sequence is alsoincluded within the scope of the present invention.

The coding portion of a nucleotide sequence encoding a peptide or apolypeptide begins with a start codon encoding Methionine which thusbecomes the N-terminal amino acid of the primary translation product. Aspart of post-translational processes, the N-terminal Methionine isfrequently cleaved off, e.g. by a Methionine aminopeptidase which is aubiquitous enzyme. In such a case, the primary translation product maygive rise to a mixture comprising members without N-terminal Methionineand members retaining this amino acid as N-terminus. It is also possiblethat the form of the enzyme without N-terminal Methionine is the onlyone which can be isolated. However, the amino acid sequences of thewild-type T7 polymerase and the T7 variants according to the inventionare described in the sequence listing including N-terminal Methionine.But the present invention also encompasses the T7 variants which do notinclude N-terminal Methionine.

For purposes of shorthand designation of T7 polymerase variantsdescribed herein, it is noted that for each mutation a number refers tothe amino acid residue/position along the reference amino acid sequenceof the wild-type T7 polymerase protein given in SEQ ID NO:2. Amino acididentification uses the three-letter abbreviations as well as thesingle-letter alphabet of amino acids, i.e., Asp D Aspartic acid, Ile IIsoleucine, Thr T Threonine, Leu L Leucine, Ser S Serine, Tyr YTyrosine, Glu E Glutamic acid, Phe F Phenylalanine, Pro P Proline, His HHistidine, Gly G Glycine, Lys K Lysine, Ala A Alanine, Arg R Arginine,Cys C Cysteine, Trp W Tryptophan, Val V Valine, Gln Q Glutamine, Met MMethionine, Asn N Asparagine. An amino acid at a particular position inan amino acid sequence may be given by its three-letter abbreviation anda number. For example, “Cys723” or “C723” denote the Cysteine residue atamino acid position 723 in SEQ ID NO:2. In any T7 mutant and/or T7variant disclosed herein, a substitution by a different amino acid maybe given as the three-letter abbreviation added after the numberindicating the position. For example, “Cys723Ser” (=[Cys723Ser]) or“C7235” (=[C723S]) denotes the substitution of Cysteine (Cys) atposition 723 in SEQ ID NO:2 by Serine (Ser) (see #6 of Table 3). ACys723Ser (=C723S) substitution results in an amino acid sequence asgiven in SEQ ID NO:12, encoded by the nucleotide sequence of SEQ IDNO:11. Exemplary further amino acid substitutions according to theinstant disclosure are disclosed in Table 1a below (see Example 1).According to some embodiments, variants may include a plurality (forexample 2 to 4) amino acid substitutions. Exemplary embodiments mayinclude amino acid substitution combinations having one or moremutations as listed in Table 1a and Table 1b.

The term “polypeptide” or “protein” denotes a polymer composed of aplurality of amino acid monomers joined by peptide bonds. According tosome embodiments, the polymer may comprise 50 or more monomers. Forexample, polypeptide or protein according to the instant inventioncomprises a T7 variant. A “peptide bond” is a covalent bond between afirst amino acid and a second amino acid in which the α-amino group ofthe first amino acid is bonded to the α-carboxyl group of the secondamino acid.

A “multimer” in the context of the present invention is understood asbeing a conjugate formed by covalently linking two or more (=a pluralityof) members, each member being a polypeptide with one or more reactiveand sterically accessible thiol groups (e.g. but not limited to Cysresidues). After formation of the conjugate, two or more members arelinked by at least one disulfide (—S—S—) bridge. Adjacent members arelinked by one or more disulfide bridge(s), and members may be linked toother members by additional disulfide bridges. A conjugate withidentical members (i.e. members of the same species of polypeptide) isalso referred to as a homomultimer. Depending on the number ofindividual members linked in the conjugate, the multimer may be referredto as a dimer, trimer, tetramer, pentamer, hexamer, etc. Aheteromultimer contains at least two different species of polypeptide.

A “reducing agent with a thiol group”, also referred to as a “—SHreagent” denotes a reducing agent capable of preventing the formation ofa disulfide bond of two —SH group-containing residues of one or morepolypeptides. For example, according to the instant disclosure, thereducing agent with a thiol group is capable of preventing multimerformation of a T7 polypeptide with a further T7 polypeptide or thepolypeptide of another species containing an —SH group. Exemplary —SHreagents within this definition include, but are not limited to,mercaptoethanol, dithiothreitol (DTT), dithioerythritol (DTE).

Illustrative T7 variants of the instant invention also comprise fusionproteins with an affinity tag such as, but not limited to, a Histidinetag (His-tag). A His-tag is an amino acid sequence containing several,for example, 3 to 7 consecutive Histidines. An illustrative embodimentof the instant disclosure include 6 consecutive Histidines. In a His-tagsequence the Histidines represent the essential portion. Butfacultatively there are few additional amino acids comprised in theHis-tag. For example, a N-terminal T7 sequence including a His-tag cancomprise the sequence N-Met His His His His His His Gly Ser-. Forexample, SEQ ID NO:74 comprises an illustrative embodiment of theinstant disclosure including the foregoing amino acid sequence. In thepresent exemplary His-tag the amino acids Gly and Ser form a linker tothe N-terminus of the T7 variant. The linker amino acids are part of theHis-tag and typically arise as a cloning artifact of the nucleotidesequence encoding the His-tag (e.g. SEQ ID NO:73). The linker sequencein the His-tag may comprise 1 to 5 amino acids, for example, althoughthe linker sequence may comprise greater than 5 amino acids.

According to embodiments of the instant invention, the N-terminalMethionine of a T7 variant may be replaced by a His-tag. Alternatively,the N-terminal sequence of the T7 variant, according to embodiments ofthe instant invention, may be extended by the His-tag. In such a case,the N-terminus of the primary translation product of the T7 variantcomprises a N-terminal Methionine followed by the His-tag, followed bythe Methionine encoded by the start codon of the original T7 encodingnucleotide sequence.

Purification of a His-tagged T7 wild-type or variant polypeptide may beefficiently performed by immobilized metal affinity chromatography, suchas is employed in the purification of recombinant proteins containing ashort affinity-tag consisting of Histidine residues (His-tag).Immobilized metal-affinity chromatography (described by Porath, J., etal., Nature 258 (1975) 598-599, for example) is based on the interactionbetween a transition metal ion (Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) immobilized on aparticulate metal chelating affinity matrix and specific amino acid sidechains. Histidine is the amino acid that exhibits the strongestinteraction with immobilized metal ion matrices, as electron donorgroups on the Histidine imidazole ring readily form coordination bondswith the immobilized transition metal.

A “vector” is a DNA which can comprise, i.e. carry, and maintain a DNAfragment according to the instant invention, including, for example,phages and plasmids. These terms are generally understood by those ofskill in the art of genetic engineering for example. The term“expression cassette” denotes a nucleotide sequence encoding apre-protein, operably linked to a promoter and a terminator. As forvectors containing an expression cassette, the terms “vector” and“expression vector” are used as synonyms.

The term “oligonucleotide” is used for a nucleic acid molecule, DNA (orRNA), generally having less than 100 nucleotides in length. Anoligonucleotide may be, for example, about 75, about 50 or even lessnucleotides in length.

“Transformation” means introducing DNA into an organism, i.e. a hostorganism, so that the DNA is replicable, either as an extrachromosomalelement or by chromosomal integration.

The term “expression” and the verb “to express” denote transcription ofDNA sequences and/or the translation of the transcribed mRNA in a hostorganism resulting in a pre-protein, i.e. not includingpost-translational processes.

A “promoter” is a regulatory nucleotide sequence that stimulatestranscription. These terms are understood by those of skill in the artof genetic engineering and the like, for example. Like a promoter, a“promoter element” stimulates transcription but constitutes asub-fragment of a larger promoter sequence.

The term “operably linked” refers to the association of two or morenucleic acid fragments on a single vector so that the function of one isaffected by the other. For example, a promoter is operably linked with acoding sequence, i.e. a nucleotide sequence encoding a protein or apre-protein, when it is capable of affecting the expression of thatcoding sequence, i.e., that the coding sequence is under thetranscriptional control of the promoter.

An objective of the present invention was to provide new mutants of T7RNA polymerase comprising no, or substantially reduced, tendency to formintramolecular disulfide bonds and generate multimers. High resolutionx-ray structures of T7 RNA polymerase were carefully inspected for theidentification of cysteine residues in the protein structures which mayallow for introduction of mutations therein.

According to embodiments of the instant disclosure, and as disclosedherein, one or more identified cysteine residues were replaced by Serineresidues. Cys-Ser substitutions were initially created as singlemutations and thereafter, several different (up to six) Cys-Sersubstitutions were combined in various separate coding sequences ofembodiments of the T7 variants disclosed herein.

The designed T7 variants were synthesized, cloned, expressed andpurified. The activity and the capability of the mutant enzymes to formmultimers were examined and compared with the wild-type enzyme. It wassurprisingly found that not all but only certain variants have asubstantially reduced tendency to form disulfide-linked intramolecularhomomultimers; that is to say, only certain T7 variants remain monomericupon long term storage, even in the presence of oxygen, without havingthe requirement to be treated repeatedly with SH-reagents.

It was another surprising finding that Cys-Ser substitutions identifiedby the present invention do not seem to have a negative impact on RNApolymerase activity. To the contrary, Cys-Ser substitutions according tothe invention can even be combined with further amino acidsubstitutions, e.g. with mutations which increase thermal stability ofT7 RNA polymerase.

According to embodiments of the instant invention, a polypeptidecomprising an improved variant of T7 RNA polymerase (T7 variant) isprovided, the T7 variant having the properties of (i) DNA-dependent RNApolymerase activity, and (ii) a different composition of amino acidscompared to the 883-amino acid T7 RNA polymerase polypeptide of SEQ IDNO:2 (wild-type reference). According to some such embodiments, theimprovement of the T7 variant may be attributed to the absence inaqueous solution of homomultimer formation as a result of anintermolecular disulfide bond(s), wherein at a position selected from510 and 530 of the T7 variant, numbered from the N-terminus of thewild-type reference, a Cysteine residue is present, and wherein the T7variant comprises one or more amino acid substitution(s) of which one(or the first) is at position 723 where Serine substitutes for theCysteine residue (Cys723Ser).

Experimental analysis involved the introduction, at selected positions,of amino acid substitutions in the T7 polypeptide. For example, theamino acids Cysteine and Serine differ only in that the sulfur atom ofthe former is replaced by oxygen in the latter. However, theelectronegativity of the oxygen atom provides the side chain of Serinewith an increased polar effect, when compared to the SH-group containingside chain of Cysteine. As such, one or more Cys-Ser substitutionschange the physico-chemicals properties of a T7 variant.

One method for protein engineering includes the modification ofenzyme-encoding nucleotide sequences, but requires knowledge of thestructure of an enzyme and detailed biochemical data concerning theprinciples underlying its function and stability. Examples ofimprovements in protein properties, according to embodiments of theinstant disclosure, include enhanced specificity, altered substratespectrum, and thermostability, for example.

In yet more detail, the present disclosure embodies the following items.

-   -   1. An aqueous solution in which a reducing agent with a thiol        group is absent, the aqueous solution comprising a polypeptide,        the polypeptide comprising a variant of T7 RNA polymerase (T7        variant), the T7 variant having the properties of (i)        DNA-dependent RNA polymerase activity, and (ii) a different        composition of amino acids compared to the 883-amino acid T7 RNA        polymerase polypeptide of SEQ ID NO:2 (wild-type reference),        -   wherein at a position selected from 510 and 530 of the T7            variant, numbered from the N-terminus of the wild-type            reference, a Cysteine residue is present,        -   wherein the T7 variant comprises one or more amino acid            substitution(s) of which one (or the first) is at position            723 where Serine substitutes for the Cysteine residue            (Cys723Ser),        -   and wherein in the aqueous solution homomultimer formation            of the T7 variant as a result of one or more intermolecular            disulfide bond(s) is absent.    -   2. The aqueous solution according to item 1, wherein in the        polypeptide the number of substituted amino acids in the T7        variant is 1 to 10.    -   3. The aqueous solution according to item 2, wherein in the        polypeptide the T7 variant further comprises a Cysteine-Serine        substitution selected from the group consisting of Cys125Ser,        Cys347Ser, Cys492Ser, Cys515Ser, and Cys839Ser.    -   4. The aqueous solution according to any of the items 2 and 3,        wherein in the polypeptide the T7 variant further comprises an        amino acid substitution selected from the group consisting of        Val426Leu, Val426Ile, Val426Phe, Ser633Val, Ser633Met,        Val650Leu, Thr654Leu, Ala702Val, and Val795Ile.    -   5. The aqueous solution according to item 4, wherein in the        polypeptide the T7 variant comprises the amino acid        substitutions Val426Leu, Val650Leu, Ala702Val, and Val795Ile.    -   6. The aqueous solution according to item 4, wherein in the        polypeptide the T7 variant comprises the amino acid        substitutions Val426Leu, Ala702Val, and Val795Ile.    -   7. The aqueous solution according to any of the items 1 to 6,        wherein the polypeptide comprises the T7 variant and an        N-terminal His-tag.    -   8. The aqueous solution according to any of the items 1 to 7,        wherein the polypeptide comprises the T7 variant with an        N-terminal Methionine.    -   9. A method to produce an aqueous solution with a polypeptide        comprising a variant of T7 RNA polymerase (T7 variant), the T7        variant having the properties of (i) DNA-dependent RNA        polymerase activity, and (ii) a different composition of amino        acids compared to the 883-amino acid T7 RNA polymerase        polypeptide of SEQ ID NO:2 (wild-type reference), wherein in the        aqueous solution homomultimer formation as a result of one or        more intermolecular disulfide bond(s) is absent, the method        comprising the steps of        -   (a) providing the T7 variant by substituting in the            wild-type reference Cys723, numbered from the N-terminus,            with Serine (Cys723Ser);        -   (b) optionally further including in the T7 variant one or            more amino acid substitution(s) selected from the group            consisting of Cys125Ser, Cys347Ser, Cys492Ser, Cys515Ser,            and Cys839Ser;        -   (c) optionally further including in the T7 variant one or            more amino acid substitution(s) selected from the group            consisting of Val426Leu, Val426Ile, Val426Phe, Ser633Val,            Ser633Met, Val650Leu, Thr654Leu, Ala702Val, and Val795Ile;        -   (d) reverse-transcribing the amino acid sequence of the            polypeptide, the polypeptide comprising the T7 variant            obtained in steps (a), (b), and (c), thereby obtaining a            nucleotide sequence encoding the polypeptide;        -   (e) expressing a nucleic acid molecule comprising the            nucleotide sequence of step (d) in an expression system, and            subsequently purifying the expressed polypeptide from the            expression system by way of chromatography using an aqueous            mobile phase in which a reducing agent with a thiol group is            absent;        -   thereby obtaining in step (e) an aqueous solution with the            polypeptide comprising the T7 variant.    -   10. The method according to item 9, wherein the aqueous solution        obtained in step (e) is stored in the absence of a reducing        agent with a thiol group.    -   11. The method according to any of the items 9 and 10, wherein        an amount of about 1 μg to 3 μg of the purified T7 variant        protein from the aqueous solution is detectably free of        homomultimers as determined by SDS polyacrylamide        electrophoresis and staining of the electrophoresed gel with the        Simply Blue Safe Stain Kit from Invitrogen.    -   12. A method to synthesize a RNA molecule, comprising the steps        of        -   (a) providing an aqueous solution with a variant polypeptide            of T7 RNA polymerase (T7 variant) according to any of the            items 1 to 8;        -   (b) providing a template DNA molecule comprising a T7            promoter, the T7 promoter being functionally linked to a            target nucleotide sequence to be transcribed;        -   (c) contacting in the aqueous solution the template DNA of            step (b) with the T7 variant of step (a) in the presence of            ribonucleoside triphosphates, thereby forming a reaction            mixture;        -   (d) incubating the reaction mixture under conditions            permitting RNA polymerase activity;        -   thereby synthesizing the RNA molecule.    -   13. A reaction mixture comprising an aqueous solution with a        variant polypeptide of T7 RNA polymerase (T7 variant) according        to any of the items 1 to 8, wherein the aqueous solution further        comprises a template DNA molecule comprising a T7 promoter        functionally linked to a target nucleotide sequence to be        transcribed, and ribonucleoside triphosphates.    -   14. A kit comprising, in separate containers, an aqueous        solution with a variant polypeptide of T7 RNA polymerase (T7        variant) according to any of the items 1 to 8 and a buffer with        one or more ribonucleoside triphosphates.    -   15. A polypeptide comprising an improved variant of T7 RNA        polymerase (T7 variant), the T7 variant having the properties        of (i) DNA-dependent RNA polymerase activity, and (ii) a        different composition of amino acids compared to the 883-amino        acid T7 RNA polymerase polypeptide of SEQ ID NO:2 (wild-type        reference), and the improvement of the T7 variant being the        absence in aqueous solution of homomultimer formation as a        result of an intermolecular disulfide bond(s),        -   wherein at a position selected from 510 and 530 of the T7            variant, numbered from the N-terminus of the wild-type            reference, a Cysteine residue is present, and        -   wherein the T7 variant comprises one or more amino acid            substitution(s) of which one (or the first) is at position            723 where Serine substitutes for the Cysteine residue            (Cys723Ser).    -   16. The polypeptide according to item 15, wherein the number of        substituted amino acids in the T7 variant is 1 to 10.    -   17. The polypeptide according to item 16, wherein the T7 variant        further comprises a Cysteine-Serine substitution selected from        the group consisting of Cys125Ser, Cys347Ser, Cys492Ser,        Cys515Ser, and Cys839Ser.    -   18. The polypeptide according to any of the items 15 to 17,        wherein the aqueous solution is detectably free of homomultimers        as determined by SDS polyacrylamide electrophoresis of up to        about 3 μg protein (preferred about 2 μg to about 3 μg protein,        more preferred up to about 2.9 μg) of the T7 variant polypeptide        in purified form, and staining of the electrophoresed gel with        the Simply Blue Safe Stain Kit (Invitrogen).    -   19. The polypeptide according to any of the items 16 to 18,        wherein the T7 variant further comprises an amino acid        substitution selected from the group consisting of Val426Leu,        Val426Ile, Val426Phe, Ser633Val, Ser633Met, Val650Leu,        Thr654Leu, Ala702Val, and Val795Ile.    -   20. The polypeptide according to item 19, wherein the T7 variant        comprises the amino acid substitutions Val426Leu, Val650Leu,        Ala702Val, and Val795Ile.    -   21. The polypeptide according to item 19, wherein the T7 variant        comprises the amino acid substitutions Val426Leu, Ala702Val, and        Val795Ile.    -   22. The polypeptide according to any of the items 15 to 21,        wherein the polypeptide comprises the T7 variant and an        N-terminal His-tag.    -   23. The polypeptide according to any of the items 15 to 22,        wherein the polypeptide comprises the T7 variant with an        N-terminal Methionine.    -   24. The polypeptide according to any of the items 15 to 23 in an        aqueous solution.    -   25. The polypeptide according to item 24, wherein in the aqueous        solution a reducing agent with a thiol group is absent.    -   26. A method to produce a polypeptide comprising an improved        variant of T7 RNA polymerase (T7 variant), the T7 variant having        the properties of (i) DNA-dependent RNA polymerase activity,        and (ii) a different composition of amino acids compared to the        883-amino acid T7 RNA polymerase polypeptide of SEQ ID NO:2        (wild-type reference), and the improvement of the T7 variant        being the absence in aqueous solution of homomultimer formation        as a result of an intermolecular disulfide bond(s), the method        comprising the steps of        -   (a) providing the T7 variant by substituting in the            wild-type reference Cys723, numbered from the N-terminus,            with Serine (Cys723Ser);        -   (b) optionally further including in the T7 variant one or            more amino acid substitution(s) selected from the group            consisting of Cys125Ser, Cys347Ser, Cys492Ser, Cys515Ser,            and Cys839Ser;        -   (c) optionally further including in the T7 variant one or            more amino acid substitution(s);        -   (d) reverse-transcribing the amino acid sequence of the            polypeptide, the polypeptide comprising the T7 variant            obtained in steps (a), (b), and (c), thereby obtaining a            nucleotide sequence encoding the polypeptide;        -   (e) expressing a nucleic acid molecule comprising the            nucleotide sequence of step (d) in an expression system, and            isolating the expressed polypeptide from the expression            system;        -   thereby producing the polypeptide.    -   27. The method according to item 26, wherein in step (c) the one        or more amino acid substitution(s) is/are selected from the        group consisting of Val426Leu, Val426Ile, Val426Phe, Ser633Val,        Ser633Met, Val650Leu, Thr654Leu, Ala702Val, and Val795Ile.    -   28. A method to produce a nucleic acid molecule with a        nucleotide sequence encoding an improved variant of T7 RNA        polymerase (T7 variant), the T7 variant having the properties        of (i) DNA-dependent RNA polymerase activity, and (ii) a        different composition of amino acids compared to the 883-amino        acid T7 RNA polymerase polypeptide of SEQ ID NO:2 (wild-type        reference), and the improvement of the T7 variant being the        absence in aqueous solution of homomultimer formation as a        result of an intermolecular disulfide bond(s), the method        comprising the steps of        -   (a) reverse transcribing an amino acid sequence of a            polypeptide according to any of the items 15 to 23 or an            amino acid sequence of a polypeptide obtainable by the            method according to item 26 or item 27, thereby obtaining a            nucleic acid sequence; followed by        -   (b) synthesizing a nucleic acid molecule with the nucleic            acid sequence obtained after performing step (a);        -   thereby producing the nucleic acid molecule encoding the T7            variant.    -   29. A nucleic acid molecule comprising a nucleotide sequence        selected from a member of the group consisting of SEQ ID NOs: 3,        5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37,        39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69,        and 71.    -   30. An expression vector comprising one or more nucleotide        sequences capable of controlling transcription and/or        translation and functionally linked to (a) a nucleic acid        molecule obtainable by the method of item 28, or (b) a nucleic        acid according to item 29.    -   31. A host organism capable of recombinant expression of a        polypeptide, wherein the host organism is transformed with an        expression vector according to item 30.    -   32. The host organism according to item 31, wherein the host        organism is Escherichia coli.    -   33. A method to synthesize a RNA molecule, comprising the steps        of        -   (a) providing a variant polypeptide of T7 RNA polymerase (T7            variant) according to any of the items 15 to 23;        -   (b) providing a template DNA molecule comprising a T7            promoter, the T7 promoter being functionally linked to a            target nucleotide sequence to be transcribed;        -   (c) contacting in aqueous solution the template DNA of            step (b) with the T7 variant of step (a) in the presence of            ribonucleoside triphosphates, thereby forming a reaction            mixture;        -   (d) incubating the reaction mixture under conditions            permitting RNA polymerase activity;        -   thereby synthesizing the RNA molecule.    -   34. The method according to item 33, wherein in any of the steps        (a), (b), (c), and (d) a reducing agent with a thiol group is        absent.    -   35. The method according to item 33, wherein in any of the steps        (a), (b), (c), and (d) a reducing agent selected from        mercaptoethanol, dithiothreitol, dithioerythritol is absent.    -   36. A reaction mixture comprising in aqueous solution a template        DNA molecule comprising a T7 promoter functionally linked to a        target nucleotide sequence to be transcribed, ribonucleoside        triphosphates, and a variant polypeptide of T7 RNA polymerase        (T7 variant) according to any of the items 15 to 23, and wherein        in the reaction mixture a reducing agent with a thiol group is        absent.    -   37. A reaction mixture comprising in aqueous solution a template        DNA molecule comprising a T7 promoter functionally linked to a        target nucleotide sequence to be transcribed, ribonucleoside        triphosphates, and a variant polypeptide of T7 RNA polymerase        (T7 variant) according to any of the items 15 to 23, and wherein        in the reaction mixture a reducing agent selected from        mercaptoethanol, dithiothreitol, dithioerythritol is absent.    -   38. A kit comprising, in separate containers, a variant        polypeptide of T7 RNA polymerase (T7 variant) according to any        of the items 15 to 23 and a buffer with one or more        ribonucleoside triphosphates, wherein the T7 variant is in        aqueous solution, wherein in the aqueous solution mixture a        reducing agent with a thiol group is absent.    -   39. A kit comprising, in separate containers, a variant        polypeptide of T7 RNA polymerase (T7 variant) according to any        of the items 15 to 23 and a buffer with one or more        ribonucleoside triphosphates, wherein the T7 variant is in        aqueous solution, wherein in the aqueous solution mixture a        reducing agent selected from mercaptoethanol, dithiothreitol,        dithioerythritol is absent.    -   40. A variant T7 RNA polymerase polypeptide having DNA-dependent        RNA polymerase activity and an amino acid sequence different        from SEQ ID NO.:2, the variant including a Cysteine residue at        amino acid position between 510 and 530, numbered from the        N-terminus of SEQ ID NO.:2, and a Serine residue substitution        for the Cysteine residue at amino acid position 723, numbered        from the N-terminus of SEQ ID NO.:2, wherein when in the aqueous        solution the variant is devoid of homomultimer formation of        intermolecular disulfide bond(s).    -   41. The variant of claim 40, wherein the variant comprises at        least 2 and less than or equal to 10 amino acid substitutions as        compared to SEQ ID NO.:2.    -   42. The variant of claim 40, wherein the variant further        comprises a Cysteine-Serine substitution selected from the group        consisting of Cys125Ser, Cys347Ser, Cys492Ser, Cys515Ser, and        Cys839Ser.    -   43. The variant of claim 40, wherein the variant further        comprises an amino acid substitution selected from the group        consisting of Val426Leu, Val426Ile, Val426Phe, Ser633Val,        Ser633Met, Val650Leu, Thr654Leu, Ala702Val, and Val795Ile.    -   44. The variant of claim 40, wherein the variant comprises amino        acid substitutions Val426Leu, Val650Leu, Ala702Val, and        Val795Ile.    -   45. The variant of claim 40, wherein the variant comprises amino        acid substitutions Val426Leu, Ala702Val, and Val795Ile.    -   46. The variant of claim 40, wherein an N-terminal His-tag is        linked to the variant.    -   47. The variant of claim 40, wherein an N-terminal Methionine is        linked to the variant.

The following examples, sequence listing, and figures are provided forthe purpose of demonstrating various embodiments of the instantdisclosure and aiding in an understanding of the present disclosure, thetrue scope of which is set forth in the appended claims. These examplesare not intended to, and should not be understood as, limiting the scopeor spirit of the instant disclosure in any way. It should also beunderstood that modifications can be made in the procedures set forthwithout departing from the spirit of the disclosure.

EXAMPLES Example 1 Design of Amino Acid Exchange Mutations in the T7Polypeptide

X-ray structures of T7 RNA polymerase deposited in the Protein Data Bank(http://www.wwpdb.org/pdb; codes: 1cez [referring to Cheetham, G. M. T.,et al., Nature 399 (1999) 80-83], and 1s77 [referring to Yin, Y. W., andSteitz, T. A., Cell 116 (2004) 393-404]) were inspected to identifycandidate sites for the introduction of mutations. To this end, cysteineresidues were localized in three dimensional models and Cys residues onthe surface of the T7 polypeptide which in principle could be accessiblefor the formation of intramolecular disulfide bonds were identified.Further, candidate amino acid residues were identified with the goal inmind to increase the stability of the protein.

Selected positions of the T7 wild-type amino acid sequence (according toSEQ ID NO:2) are shown in Table 1a which show candidate Cysteines. Table1b provides amino acid substitution mutations expected to increase thestability of the T7 polymerase protein. The underlying rationale of thedesign of the mutations is also indicated, in order to increase thestability of a variant T7 polypeptide versus the wildtype reference.Thus, most of the substituting amino acids were selected either to fillhydrophobic cavities in the core or to stabilize loops located at thesurface of the enzyme.

TABLE 1a Amino Acid Mutations of T7RNA Polymerase: Design of an Enzymewith Reduced Tendency to Form Multimers Linked by IntramolecularDisulfide Bridges. Cys residue in WT T7 polypeptide, position in aminoacid sequence Predicted Proposed (SEQ ID NO: 2) localization mutation125 surface Ser 216 buried — 271 buried — 347 surface Ser 467 buried —492 surface Ser 510 buried — 515 surface Ser 530 buried — 540 buried —723 surface Ser 839 surface Ser

TABLE 1b Amino Acid Mutations of T7 RNA Polymerase: Design of an Enzymewith Increased Thermostability. Amino acid, WT Position MutationRationale Val 426 Leu, Ile, Phe, Fill cavity in protein Trp core Ser 633Val, Leu, Met Stabilize loop Val 650 Leu Stabilize loop Thr 654 LeuStabilize loop Ala 702 Val Fill cavity in protein core Val 795 Ile Fillcavity in protein core

In order to provide a coding sequence for any of the T7 mutantspresented herein, the nucleotide sequence of SEQ ID NO:1 encoding the T7wild type reference polypeptide was used as a basis. The nucleotidecodons corresponding to the amino acid residues at the positionsindicated in Tables 1a and 1b were mutated, in order to encode thechanged amino acid at the respective position. Mutations were preferablydesigned in accordance with the codon usage bias of E. coli class IIgenes (as described in Hénaut, A., and Danchin, A., Analysis andPredictions from Escherichia coli sequences. Escherichia coli andSalmonella, Vol. 2, Ch. 114 (1996) 2047-2066, Neidhardt FC ed., ASMpress, Washington, D.C.), as given in Table 2.

TABLE 2 Codon Usage in E. Coli. Amino Class Amino Class acid Codon I IIIII acid Codon I II III Phe TTT 55.09 29.08 67.14 Leu CTT 9.7 5.56 19TTC 44.91 70.92 32.86 CTC 10.4 8.03 9.04 Leu TTA 10.99 3.44 20.09 CTA3.09 0.83 6.81 TTG 13.02 5.47 15.05 CTG 52.79 76.67 29.99 Ser TCT 13.2632.41 19.63 Pro CCT 13.71 11.23 28.3 TCC 15.02 26.56 11.34 CCC 11.191.63 16.26 TCA 10.83 4.79 22.09 CCA 18.63 15.25 31.5 TCG 16.88 7.39 10.6CCG 56.47 71.89 23.94 Tyr TAT 54.42 35.23 69.6 His CAT 56.8 29.77 61.69TAC 45.58 64.77 30.4 CAC 43.2 70.23 38.31 Stop TAA Gln CAA 33.4 18.6537.06 TAG CAG 66.6 81.35 62.94 Cys TGT 40.9 38.85 55.71 Arg CGT 38.9964.25 26.05 TGC 59.1 61.15 44.29 CGC 42.23 32.97 21.94 Stop TGA CGA 5.521.07 12.8 Trp TGG 100 100 100 CGG 8.97 0.8 13.62 Ile ATT 51.2 33.4947.57 Val GTT 23.74 39.77 34.33 ATC 44.37 65.94 26.65 GTC 22.48 13.4518.95 ATA 4.43 0.57 25.78 GTA 14.86 19.97 21.78 Met ATG 100 100 100 GTG38.92 26.81 24.94 Thr ACT 14.85 29.08 26.83 Ala GCT 14.52 27.54 22.86ACC 46.83 53.6 24.45 GCC 27.62 16.14 23.67 ACA 10.52 4.67 27.93 GCA19.63 24.01 31.27 ACG 27.81 12.65 20.8 GCG 38.23 32.3 22.19 Asn AAT40.87 17.25 64.06 Asp GAT 62.83 46.05 70.47 AAC 59.13 82.75 35.94 GAC37.17 53.95 29.53 Lys AAA 75.44 78.55 72.21 Glu GAA 68.33 75.35 66.25AAG 24.56 21.45 27.79 GAG 31.67 24.65 33.75 Ser AGT 13.96 4.52 18.73 GlyGGT 32.91 50.84 31.79 AGC 30.04 24.33 17.61 GGC 43.17 42.83 24.51 ArgAGA 1.75 0.62 15.63 GGA 9.19 1.97 24.75 AGG 1.54 0.29 9.96 GGG 14.744.36 18.95

The genes which served as the basis for the data in Table 2 wereclustered by using factorial correspondence analysis into three classes.Class I contains genes involved in most metabolic processes. Class IIgenes correspond to genes highly and continuously expressed duringexponential growth. Class III genes are implicated in horizontaltransfer of DNA. One can see that the distribution of codons in classIII genes is more or less even, whereas it is extremely biased in classII genes (in particular, codons terminated in A are selected against).

The mutations on the codon level which were introduced in the T7 codingsequence are shown in Table 3.

TABLE 3 T7 Polymerase and Variants Thereof. T7 enzyme/ # variantWT codon Mutated codon SEQ ID NO:  1 Wild-type —  1, 2  2 Cys125Ser TGCAGC  3, 4  3 Cys347Ser TGT AGC  5, 6  4 Cys492Ser TGC AGC  7, 8  5Cys515Ser TGC AGC  9, 10  6 Cys723Ser TGC AGC 11, 12  7 Cys839Ser TGTAGC 13, 14  8 Cys125Ser TGC AGC 15, 16 Cys347Ser TGT AGC Cys492Ser TGCAGC Cys515Ser TGC AGC Cys723Ser TGC AGC Cys839Ser TGT AGC  9 Val426LeuGTT CTG 17, 18 10 Cys723Ser TGC AGC 19, 20 Val426Leu GTT CTG 11Val4261Ie GTT ATC 21, 22 12 Cys723Ser TGC AGC 23, 24 Val426Ile GTT ATC13 Val426Phe GTT TTC 25, 26 14 Cys723Ser TGC AGC 27, 28 Val426Phe GTTTTC 15 Ser633Met TCA ATG 29, 30 16 Cys723Ser TGC AGC 31, 32 Ser633MetTCA ATG 17 Val650Leu GTG CTG 33, 34 18 Cys723Ser TGC AGC 35, 36Val650Leu GTG CTG 19 Thr654Leu ACC CTG 37, 38 20 Cys723Ser TGC AGC39, 40 Thr654Leu ACC CTG 21 Ala702Val GCT GTT 41, 42 22 Cys723Ser TGCAGC 43, 44 Ala702Val GCT GTT 23 Val795Ile GTA ATC 45, 46 24 Cys723SerTGC AGC 47, 48 Val795Ile GTA ATC 25 Ala702Val GCT GTT 49, 50 Val795IleGTA ATC 26 Cys723Ser TGC AGC 51, 52 Ala702Val GCT GTT Val795Ile GTA ATC27 Val426Leu GTT CTG 53, 54 Ala702Val GCT GTT 28 Cys723Ser TGC AGC55, 56 Val426Leu GTT CTG Ala702Val GCT GTT 29 Val426Leu GTT CTG 57, 58Val795Ile GTA ATC 30 Cys723Ser TGC AGC 59, 60 Val426Leu GTT CTGVal795Ile GTA ATC 31 Val426Leu GTT CTG 61, 62 Ala702Val GCT GTTVal795Ile GTA ATC 32 Cys723Ser TGC AGC 63, 64 Val426Leu GTT CTGAla702Val GCT GTT Val795Ile GTA ATC 33 Cys125Ser TGC AGC 65, 66Cys347Ser TGT AGC Cys492Ser TGC AGC Cys515Ser TGC AGC Cys723Ser TGC AGCCys839Ser TGT AGC Val426Leu GTT CTG Ala702Val GCT GTT Val795Ile GTA ATC34 Val426Leu GTT CTG 67, 68 Val650Leu GTG CTG Ala702Val GCT GTTVal795Ile GTA ATC 35 Cys723Ser TGC AGC 69, 70 Val426Leu GTT CTGVal650Leu GTG CTG Ala702Val GCT GTT Val795Ile GTA ATC 36 Cys125Ser TGCAGC 71, 72 Cys347Ser TGT AGC Cys492Ser TGC AGC Cys515Ser TGC AGCCys723Ser TGC AGC Cys839Ser TGT AGC Val426Leu GTT CTG Val650Leu GTG CTGAla702Val GCT GTT Val795Ile GTA ATC

On the amino acid level, the T7 variants are shown in the even-numberedSEQ ID NOs: 4 to 72.

The nucleotide sequences encoding the mutated T7 polypeptides which wereexpressed in E. coli are shown in the uneven-numbered SEQ ID NOs: 3 to71. The nucleotide sequences are represented including the start codonsfor N-terminal Methionine but without any other additional artificialN-terminal structures such as His-tags.

A His-tag (in the literature also referred to as a polyHis-tag) is anamino acid motif in proteins that typically consists of at least sixconsecutive His residues (His6). While the N-terminus of a T7 variant ispreferred for the addition of the His-tag, the C-terminus of thepolypeptide can serve as an alternative.

For clarification, a N-terminal His-tag can be located between theMethionine at the N-terminus of the respective variant T7 polypeptideand the subsequent amino acid according to the amino acid sequence ofSEQ ID NO:2, i.e. Asn. Alternatively, the His-tag can be appended to theN-terminal Methionine of the T7 variant. When appended at the C-terminusof the variant T7 polypeptide the His-tag forms the C-terminal aminoacids.

The T7 variants were modified further such that each polypeptidecontained a His-tag at its N-terminus to facilitate purification.

Apart from the Histidines the His-tag can additionally comprise furtheramino acids depending on the design of the nucleotide sequence encodingthe His-tag. Thus, an oligonucleotide linker with restriction sitestypically adds 1 to 5 further amino acids to the nucleotide fragmentencoding the consecutive His residues in the His-tag.

The amino acid sequences of the T7 variants of Tables 1a and 1b, and thenucleic acid sequences encoding the T7 variants are shown in thesequence listing of this disclosure. No His-tags are shown as these maydiffer sequence-wise, depending on the particular cloning vector used.However, the differences concerning the number of Histidines and thelinker sequence, according to the preferred embodiments, are notexpected to have a technical impact on the T7 variants according to theinvention.

Example 2 Cloning of Nucleic Acids Encoding Variants of T7 RNAPolymerase

All molecular biological procedures were performed according to standardmethods (Sambrook, J., Fritsch, E. F., Maniatis, T., Molecular cloning:A Laboratory Manual second Edition, B.27 (1989) Cold Spring HarborLaboratory Press NY (USA)). Nucleotide sequences encoding the wild-typeand the mutant T7 polypeptides were synthesized by a combinatorialsynthesis strategy as described (van den Brulle, J., et al.,Biotechniques 45(3) (2008) 340-343).

For expression of each of the T7 variants, the respective coding DNAsequence was cloned in appropriate expression vectors in such a way thatthe mutated T7 coding sequence is inserted in the right orientationunder the control of an appropriate promoter, preferably an induciblepromoter, particularly preferably the lac-, lacUV5-, tac- or T5promoter. Preferred expression vectors are pUC plasmids with lac- orlacUV5 promoters or pKK plasmids. For clarification an exemplary codingsequence comprises a DNA encoding a polypeptide selected from any of SEQID NOs: 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35,37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, and71, which optionally include a further modification such as a His-tag.

The synthesized genes were cloned in plasmid pUC18. The recipient strainfor transformations was E. coli XL-1 blue. Transformed clones were grownat 37° C. in LB media containing ampicillin (100 μg/ml). Plasmids wereisolated and digested using EcoRI and HindIII. The resulting fragmentswere subjected to agarose gel electrophoresis and the respective bandcorresponding to the variant T7 polymerase coding sequence wasextracted. The isolated fragments were ligated into the expressionplasmid pKKT5 (derived from pKK177-3 [Kopetzki, E., et al., Mol. Gen.Genet. 216 (1989) 149-155] by exchanging the tac-promotors with theT5-promoter derived from the plasmid pDS [Bujard, H., et al., MethodsEnzymol. 155 (1987) 416-433]) which was digested with EcoRI and HindIII.

Plasmids were transformed into E. coli UT5600 (harboring plasmidpUBS520). Clones were grown at 37° C. in LB media containing ampicillin(100 μg/ml) and kanamycin (50 μg/ml).

Example 3 Expression of Variant T7 Polymerase Polypeptides

Transformed E. coli expression strains obtained as described in Example2 were cultivated at 37° C. in LB media containing ampicillin (100μg/ml) and kanamycin (50 μg/ml). Induction of recombinant expression wasperformed at an optical density of 0.7 (measured at 578 nm) by addingIPTG in a final concentration of 1 mM. After 5 hours the cells wereharvested by centrifugation and frozen at −20° C.

Example 4 Purification of Variant T7 Polymerase Polypeptides

The following purification protocol is preferred according to theinvention. His 6-tagged wild-type T7 polypeptide and variants of T7 RNApolymerase were routinely purified to homogeneity using chromatographicmethods. Reducing additives like 2-mercaptoethanol or DTT were omittedduring the entire purification process. Frozen cells (typically 2.1 g)were suspended in 30 ml lysis buffer (50 mM Tris/HCl, pH 8.0 adjusted atroom temperature, 0.2 M NaCl, 2 mM EDTA). After incubation at roomtemperature for 15 min, the cells were sonicated. After addition of 1 mlPolymin P cell debris was removed by centrifugation at 10,000 rpm for 10min (Eppendorf centrifuge). The supernatant was dialyzed against BufferA (20 mM potassium phosphate, pH 7.7, 1 mM EDTA, 50 mM NaCl, 5%glycerol). After centrifugation the pool was applied on a S-Sepharose™ff column (1.6×10 cm) at a flow rate of 5 ml/min. Elution was performedusing a NaCl gradient (0 M to 1 M in Buffer A). Fractions were monitoredby running aliquots on a SDS gel. Fractions containing T7 RNA polymerasewere pooled. After dialysis against Buffer B (50 mM Tris/HCl, pH 8.0 (pHadjusted at 25° C.), 1 M NaCl) each enzyme solution was applied on aNi-chelating Sepharose™ ff column (4 ml). The column was washed usingBuffer B. T7 RNA polymerase was eluted in Buffer B using an imidazolegradient of 0 M to 1 M. Fractions containing the enzyme were pooled.After dialysis against storage buffer (25 mM Tris/HCl, pH 7.5 [pHadjusted at 25° C.], 10 mM NaCl, 0.1 mM EDTA) pools were stored at −20°C.

Alternative purification protocol includes the use of reducing additives(less preferred): His6-tagged wild-type T7 polymerase and T7 variantswere purified separately to homogeneity using metal chelate affinitymatrix chromatography. Typically, wet frozen cells (2.1 g) weresuspended in 30 ml Buffer C (50 mM Tris/HCl, pH 8.1 [pH adjusted at roomtemperature], 1 M NaCl). To the suspension 315 μl of a lysozyme solution(10 mg/ml) were added. After incubation at room temperature for 15 min,the cells were sonicated (6×2 min). The cell debris was removed bycentrifugation at 5,000 rpm for 15 min. A fraction of the supernatant(25 ml) was applied onto a Ni-chelating Sepharose column (1 ml). Thecolumn was washed using a modified Buffer C which additionally contained10 mM imidazole. His6-tagged polypeptides were eluted in a lineargradient (10 mM-500 mM imidazole in Buffer C). Enzyme-containingfractions were pooled. After dialysis against storage buffer (10 mMpotassium phosphate, 200 mM KCl, 0.1 mM EDTA, 30 mM mercaptoethanol as areducing additive, 50% glycerol, 0.1% Tween 20, pH 7.9) the pools werestored at −20° C.

Example 5 SDS Polyacrylamide Electrophoresis (SDS-PAGE)

Samples of T7 variants were prepared according to a procedure describedin Example 4. T7 RNA polymerase and variants thereof were analyzed bygel electrophoresis on polyacrylamide gels containing sodiumdodecylsulphate (SDS). Gradient gels (NuPAGE, 4-12%, Bis-Tris Gel,Invitrogen) were used. Typically, protein samples (0.26 mg/ml, 18 μl)were mixed with 6 μl of NuPAGE LDS (Lithium dodecyl sulfate; LDS mayalso be substituted by sodium dodecyl sulfate) sample buffer, (4×,Invitrogen). After heating for 2 min at 85° C., samples (20 μl) wereapplied on the gel. The total amount of protein in a single lane of thegel was between 1 μg and 3 μg. Gels were run in SDS Running Buffer(1×MES, Invitrogen) at 200 V for 1 hour.

Protein bands in the gels were stained using the Simply Blue Safe StainKit (Invitrogen product number LT6060) according to the instructions ofthe manufacturer. A protein molecular weight marker (Mark 12,Invitrogen) was used to determine the apparent molecular weight and toidentify monomeric and homopolymeric forms of T7 RNA polymerase and/orT7 variants.

As a reference sample, commercially available T7 RNA polymerase (withoutHis-tag) was used (wild-type T7 commercially available from RocheApplied Science, Mannheim, Germany).

Example 6 Application of Conditions for Formation of Homodimers andHigher-Order Multimers

T7 enzyme and variants were purified according to Example 4 using thepreferred procedure without a reducing agent. In order to comparetendencies to form intramolecular dimers and higher-order multimers,separate preparations of wild-type T7 polymerase (#1 in Table 3) andvariants (##2 to 8 shown in Table 3) were incubated under conditionsfavouring the formation of homomultimers. To this end, samples ofpurified enzymes at a protein concentration of 0.2 mg/ml to 0.3 mg/mlwere incubated individually at 37° C. for 16 hours in 25 mM Tris/HCl, pH7.5 (pH adjusted at 25° C.), 0.1 mM EDTA, 100 mM NaCl under otherwiseambient conditions. This includes exposure to atmospheric oxygen.

Example 7 Analysis of Homodimer Formation

The capability to form dimers was studied for wild-type and T7 variantsunder stress conditions. To this end, samples were incubated in a bufferwithout reducing reagents. Protein monomers, dimers and higher-ordermultimers were determined by SDS gel electrophoresis as described inExample 5.

Samples of wild-type T7 RNA polymerase (0.2 to 0.3 mg/ml) were incubatedat 37° C. for 16 hours in 25 mM Tris/HCl, pH 7.5 (adjusted at 25° C.),0.1 mM EDTA, 100 mM NaCl. Aliquots were removed and applied on the SDSgels under reducing conditions (10 mM DTT added) or under non-reducingconditions (DTT omitted, no further reducing agent present). As shown inFIG. 1, the enzyme sample of wild-type T7 contained monomeric anddimeric forms following incubation under non-reducing conditions (in thepresent case without DTT, lane 1). Analysis of the sample under reducingconditions (with DTT) showed that the protein was completely transformedto the monomeric form (lane 2).

With reference to FIG. 1 (and in reference to Examples 5, 6, and 7), astained SDS polyacrylamide gel is shown. The lanes designated M containthe Mark 12 size marker (Invitrogen). Lanes 1 and 2 contain wild-type T7RNA polymerase. In both lanes the prominent band of fastest migratingprotein represents monomeric T7 polypeptide. Samples of T7 RNApolymerase incubated for 16 h at 37° C. were treated or not treated withreducing reagents (DTT), and were subjected to electrophoresis. Lane 1:sample without DTT; Lane 2: sample treated with DTT. In lane 1 theregions marked with one and two asterisks indicate T7 multimers. Theregion marked with one asterisk contains only a faint smear. In theregion marked with two asterisks, distinct bands can be discerned,indicating dimers and distinct higher-order homomultimers.

Further, samples of wild-type T7 and T7 variants (0.2 to 0.3 mg/ml each)were incubated at 37° C. for 16 hours in 25 mM Tris/HCl, pH 7.5(adjusted at 25° C.), 0.1 mM EDTA, 100 mM NaCl. Aliquots were removedand applied on the SDS gels under reducing conditions or undernon-reducing conditions. After incubation a sample of each treatedpreparation was subjected to SDS polyacrylamide gel electrophoresis asdescribed in Example 5. Results are depicted in FIG. 2 (reducingreagents absent) and FIG. 3 (10 mM DTT added).

With reference to FIG. 2 (reducing reagents absent), and in reference toExamples 5, 6, and 7, a stained SDS polyacrylamide gel is shown. Thelanes designated M contain the Mark 12 size marker (Invitrogen). Noreducing agent was added prior to electrophoresis. Lane 1: wild-type T7RNA polymerase without His-tag; Lane 2: wild-type T7 RNA polymeraseincluding His6-tag; Lane 3: T7 variant #8 [Cys125Ser, Cys347Ser,Cys492Ser, Cys515Ser, Cys723Ser, Cys839Ser] including His 6-tag; Lane 4:T7 variant #6 [Cys723Ser] including His6-tag; Lane 5: T7 variant #3[Cys347Ser] including His6-tag; Lane 6: T7 variant #7 [Cys839Ser]including His6-tag; Lane 7: T7 variant #2 [Cys125Ser] includingHis6-tag; Lane 8: T7 variant #4 [Cys492Ser] including His6-tag; Lane 9:T7 variant #5 [Cys515Ser] including His6-tag; To varying extentshomodimers and higher-order homomultimers can be seen in lanes 1, 2, 5,6, 7, 8, and 9. Lanes 3 and 4 are detectably free of homodimers and/orhomomultimers.

With reference to FIG. 3 (10 mM DTT added), and in reference to Examples5, 6, and 7, a stained SDS polyacrylamide gel is shown. The lanesdesignated M contain the Mark 12 size marker (Invitrogen). 10 mM DTT wasadded prior to electrophoresis, to provide reducing conditions. Lane 1:wild-type T7 RNA polymerase without His-tag; Lane 2: wild-type T7 RNApolymerase including His6-tag; Lane 3: T7 variant #8 [Cys125Ser,Cys347Ser, Cys492Ser, Cys515Ser, Cys723Ser, Cys839Ser] includingHis6-tag; Lane 4: T7 variant #6 [Cys723Ser] including His6-tag; Lane 5:T7 variant #3 [Cys347Ser] including His6-tag; Lane 6: T7 variant #7[Cys839Ser] including His6-tag; Lane 7: T7 variant #2 [Cys125Ser]including His6-tag; Lane 8: T7 variant #4 [Cys492Ser] includingHis6-tag; Lane 9: T7 variant #5 [Cys515Ser] including His6-tag.

Each lane with wild-type T7 or T7 variant in FIGS. 1, 2 and 3 representsan amount of 2.9 μg of protein.

The most surprising finding was that among the T7 variants with Cys-Sersubstitutions tested, all those with a Cys723Ser mutation did not showdetectable traces of homodimers or any higher-order homomultimers in SDSgels under the conditions specified in Example 5. This effect was notonly observed in T7 variants with only Cys-Ser substitutions. The sameeffect was also obtained (i.e. homomultimers were detectably absent)when Cys723Ser was combined with any other substitution shown in Table3. These results strongly suggest that the Cys723Ser mutation canadvantageously be combined with other substitution mutations thusleading to variants with suppressed homomultimer formation.

It was noted that the effect of any other of the Cys-Ser substitutionmutations selected from the group consisting of Cys125Ser, Cys347Ser,Cys492Ser, Cys515Ser, and Cys839Ser was not comparable in thathomodimers or any higher-order homomultimers were detectable in varyingamounts. In SDS gels T7 variants with Cys839Ser just showed a detectablereduction of homomultimer formation but no absence of homomultimers.

As shown in FIG. 1, the Cys723Ser substitution alone is remarkablyeffective in suppressing homodimer (and homomultimer) formation.Nevertheless, accumulating more Cys-Ser substitutions in the T7polypeptide is desirable, in order to minimize formation of heterodimersand -multimers, i.e. to suppress covalent disulfide bonding with otherproteins but T7 polypeptide. In this regard, a T7 variant comprisingCys723Ser and one or more further substitutions selected from Cys125Ser,Cys347Ser, Cys492Ser, Cys515Ser, and Cys839Ser has been found to be ofgreat advantage.

Example 8 Analysis of DNA-Dependent RNA Polymerase Activity

A transcription-based non-radioactive assay (Method A) was used tomeasure the activity of purified wild-type and variants of T7 RNApolymerase obtained as described in Example 3. The enzyme activity wasmeasured in 40 μl reaction buffer (40 mM Tris/HCl, 6 mM MgCl₂, 1 mM NTP(each), 0.002% [v/v] polydocanol, 4 mM spermidine, pH 8.0, 1 μg ofplasmid pSPT18 cleaved (i.e. linearized) with Sspl). T7 wild-type or aT7 variant polymerase enzyme was added in diluted form. After incubationat 37° C. for 30 min EDTA (0.4 M, 4 μA was added to stop the reaction.

Subsequently, each reaction mix (as an aliquot of 100 μA was mixed with100 μl of SYBR Green II (diluted 1:4,000) and a volume of 1,890 μl 1×TEbuffer were added. Fluorescence was measured photometrically (excitationwavelength: 485 nm, emission wavelength 530 nm) using a fluorimeter(Cary Eclipse, Varian). As a reference enzyme commercially available T7RNA polymerase (Roche Applied Science, Roche Diagnostics GmbH, Mannheim)was used.

Alternatively (Method B), RNA quantification was done using Quant-iT RNAAssay (Invitrogen) on a LC480 Light Cycler platform (Roche AppliedScience, Roche Diagnostics GmbH, Mannheim). As a reference enzymecommercially available T7 RNA polymerase (Roche Applied Science, RocheDiagnostics GmbH, Mannheim) was used.

To test the T7 enzyme and variants thereof under non-reducingconditions, comparative analysis was done in that in each case in afirst experiment DTT was included, and in a second experiment DTT wasomitted in the reaction mix.

Following incubation as described in Example 6, the enzymatic activityof samples containing protein multimers was tested, too.

Item A in Table 4 indicates the enzymatic activity of wild-type T7polypeptide in the sample depicted in FIG. 1, lane 1 which shows thepresence of dimers and higher-order multimers to a significant extent.Item B in the table corresponds to the protein in lane 2 of FIG. 1. Asit becomes evident in the assay, enzymatic activity is lower in theabsence of any reducing agent and increases upon addition of DTT. Acomparable effect is obtained with other reducing agents with one ormore thiol groups, such as mercaptoethanol or DTE.

TABLE 4 Activity of Wild-Type T7 RNA Polymerase Under Non-Reducing andReducing Conditions. fluorescence Assay setup (arbitrary units) A noreducing agent 8.09 B 10 mM DTT in reaction mixture 12.43

The data show that the enzyme treated under non-reducing conditionsshows a significantly reduced activity. This effect is directlycorrelated to the presence of protein dimers and higher-order multimersin the enzyme preparation.

Further, the effect of Cys-Ser substitutions in variant T7 polypeptideswas determined; all assays were performed in the presence of 10 mM DTT.In each assay the concentration of the respective T7/T7 variantpolymerase was 6.7 μg/ml. Table 5 shows exemplary data of wild-type andmutant T7 polypeptide enzymatic activities. T7 designations are made inaccordance with those given in Table 3 above.

TABLE 5 Activity of Wild-Type T7 RNA Polymerase and Variant T7Polypeptides. fluorescence T7 polypeptide (arbitrary units) wild-type, #1 46.4 T7 variant # 6 45.9 T7 variant # 8 49.2

Very surprisingly, no negative impact with regards to enzyme activitywas observed even when six substitutions, i.e. Cys125Ser, Cys347Ser,Cys492Ser, Cys515Ser, Cys723Ser, and Cys839Ser were accumulated in thesame polypeptide (T7 variant #8, according to Table 3).

Example 9 Analysis of Thermostability: Half-Life Time

Further substitutions were made in the T7 polypeptide, in order toadditionally enhance thermal stability of the polypeptide. T7 variants##37 to 48 are shown in Table 6 below.

To determine the stability of wild-type T7 polymerase and T7 variantsthe half-life time was determined at 50° C. Samples of wild-type enzymeand purified variants (see Examples 3 and 4) were incubated in storagebuffer (10 mM potassium phosphate, 200 mM KCl, 0.1 mM EDTA, 30 mMmercaptoethanol, 50% glycerol, 0.1% Tween 20, pH 7.9) at 50° C. Atdifferent time points (10, 20 and 30 min) samples were taken and theresidual enzyme activity was measured as described in Example 3. Thehalf-life time T_(1/2) expressed as a number of minutes [min] means thatat this time point the activity of the respective T7 variant is 50% ofthe activity at the time point when the experiment was started, i.e. theexposure to 50° C. was applied. Table 6 summarizes results of themeasurements.

TABLE 6 Half-Life Times at 50° C. of Wild-Type T7 RNA Polymerase and T7Variants (Single Mutations and Combination Mutations). # T7 enzymeT_(1/2) [min] Reference 1 Wild-type 6.0-9.7 Single amino acidsubstitution 37 Val426Leu 25.0 38 Val426Ile 17.0 39 Ser633Met 13.0 40Val650Leu 13.0 41 Thr654Leu 13.0 42 Ala702Val 22.0 43 Val795Ile 29.0Double amino acid substitution 44 Ala702Val 22.0 Val795Ile 45 Val426Leu39.0 Ala702Val 46 Val426Leu 40.0 Val795Ile Triple amino acidsubstitution 47 Val426Leu 312.0 Ala702Val Val795Ile Quadruple amino acidsubstitution 48 Val426Leu 64.0 Val650Leu Ala702Val Val795Ile

With regards to half-life times at 50° C., the inventors observedseveral surprising effects. Firstly, there were single amino acidexchanges without noticeable impact on thermostability, i.e. mutationswhich did not cause a substantial difference compared to the wild-typereference (#1). In this first group all T7 variants with a T1/2 valuebetween 5 and 12 (including 5 and 12) were combined (not shown). Asecond group of T7 variants (not shown) was found in which the mutantshad even shorter half-life times at 50° C., compared to the wild-typereference. Additionally, mutants which had lost enzymatic activitycompletely were combined in the second group. A third group of aminoacid exchange mutations was found to enhance half-life time at 50° C.over the values found for the wild-type reference. A value greater than12 was considered as indicating a substantial increase ofthermostability in the respective T7 variant. The third group comprisesthe mutations according to ##37 to 48 as shown in Table 6.

Surprisingly, some amino acid substitutions which, according totheoretical predictions, were predicted to have a desired positiveeffect on thermostability did not lead to the expected results.

Very surprisingly, the amino acid substitutions shown in Table 6 couldbe combined with any of the Cys-Ser substitutions six substitutionsdescribed above, i.e. with Cys125Ser, Cys347Ser, Cys492Ser, Cys515Ser,Cys723Ser, and Cys839Ser, wherein variant T7 polypeptides withDNA-dependent RNA activity were obtained. Even more surprising, T7variants combining (a) either Cys723Ser or all of the above six Cys-Sersubstitutions with (b) a single, a double, a triple, or a quadrupleamino acid substitution as shown in Table 6 were found to be morethermostable than the wild-type T7 polypeptide. Combiningthermostability with a reduced tendency to form intramolecular disulfidebridges, the T7 variants ##10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30,32, 33, 35, and 36 (according to Table 3) are particularly advantageousand very much preferred.

Example 10 Determination of Protein Concentration in Solutions

Protein concentrations were determined by measuring the optical densityat 280 nm using a molar extinction coefficient of E280 nm=1.4×10⁵ M⁻¹cm⁻¹ as described (He, B., et al., Protein Expr Purif 9 (1997) 142-151).

All publications, patents and applications are hereby incorporated byreference in their entirety to the same extent as if each such referencewas specifically and individually indicated to be incorporated byreference in its entirety.

While this disclosure has been described as having an exemplary design,the present disclosure may be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the disclosure using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within the known orcustomary practice in the art to which this disclosure pertains.

1. An aqueous solution being devoid of a reducing agent with a thiolgroup, the aqueous solution comprising: a variant polypeptide of T7 RNApolymerase, the variant having DNA-dependent RNA polymerase activity andan amino acid sequence different from SEQ ID NO.:2, wherein the variantincludes a Cysteine residue at amino acid position between 510 and 530,numbered from the N-terminus of SEQ ID NO.:2, wherein the variantincludes a Serine residue substitution for the Cysteine residue at aminoacid position 723, numbered from the N-terminus of SEQ ID NO.:2, andfurther wherein in the aqueous solution the variant is devoid ofhomomultimer formation of intermolecular disulfide bond(s).
 2. Theaqueous solution according to claim 1, wherein the T7 variant comprisesat least 2 and less than or equal to 10 amino acid substitutions ascompared to SEQ ID NO.:2.
 3. The aqueous solution according to claim 2,wherein the variant further comprises a Cysteine-Serine substitutionselected from the group consisting of Cys125Ser, Cys347Ser, Cys492Ser,Cys515Ser, and Cys839Ser.
 4. The aqueous solution according to claim 2,wherein the variant further comprises an amino acid substitutionselected from the group consisting of Val426Leu, Val426Ile, Val426Phe,Ser633Val, Ser633Met, Val650Leu, Thr654Leu, Ala702Val, and Val795Ile. 5.The aqueous solution according to claim 4, wherein the variant comprisesamino acid substitutions Val426Leu, Val650Leu, Ala702Val, and Val795Ile.6. The aqueous solution according to claim 4, wherein the variantcomprises amino acid substitutions Val426Leu, Ala702Val, and Val795Ile.7. The aqueous solution according to claim 1, further comprising anN-terminal His-tag linked to the variant.
 8. The aqueous solutionaccording to claim 1, further comprising an N-terminal Methionine linkedto the variant.
 9. The aqueous solution according to claim 1, whereinthe reducing agent with the thiol group is selected from the groupconsisting of mercaptoethanol, dithiothreitol, dithioerythritol.
 10. Amethod of producing an aqueous solution devoid of a reducing agentincluding a thiol group, the solution comprising a variant of T7 RNApolymerase having DNA-dependent RNA polymerase activity and an aminoacid sequence different from SEQ ID NO:2, the variant being devoid ofhomomultimer formation of intermolecular disulfide bonds when in thesolution, the method comprising the steps of: providing the variant bysubstituting a Cysteine residue at amino acid position 723, numberedfrom the N-terminus of SEQ ID NO.:2, with a Serine residue;reverse-transcribing the amino acid sequence of the variant, therebyobtaining a nucleotide sequence encoding the variant; expressing anucleic acid molecule comprising the nucleotide sequence of the variantobtained in the step of reverse transcribing in an expression system,thereby expressing a polypeptide; and purifying the polypeptideexpressed in the step of expressing, by way of chromatography using anaqueous mobile phase devoid of a reducing agent including a thiol group,thereby obtaining the solution with the variant.
 11. The methodaccording to claim 10, wherein the solution obtained in the step ofpurifying is stored in the absence of a reducing agent with a thiolgroup.
 12. The method according to claim 11, wherein an amount of about1 to 3 μg of the variant protein obtained in the step of purifying isdetectably free of homomultimers, as determined by SDS polyacrylamideelectrophoresis and staining with a Simply Blue Safe Stain Kit.
 13. Themethod according to claim 10, wherein the nucleic acid moleculecomprising the nucleotide sequence of the variant comprises a T7promoter functionally linked to a target nucleotide sequence to betranscribed.
 14. The method according to claim 10, wherein theexpression system includes a buffer with one or more ribonucleosidetriphosphates.
 15. The method according to claim 10, wherein the variantfurther comprises a Cysteine-Serine substitution selected from the groupconsisting of Cys125Ser, Cys347Ser, Cys492Ser, Cys515Ser, and Cys839Ser.16. The method according to claim 10, wherein the variant furthercomprises an amino acid substitution selected from the group consistingof Val426Leu, Val426Ile, Val426Phe, Ser633Val, Ser633Met, Val650Leu,Thr654Leu, Ala702Val, and Val795Ile.
 17. A method of synthesizing a RNAmolecule, comprising the steps of: providing an aqueous solution devoidof a reducing agent with a thiol group, the solution having a variantpolypeptide of T7 RNA polymerase, the variant having DNA-dependent RNApolymerase activity and an amino acid sequence different from SEQ IDNO:2, the variant being devoid of homomultimer formation ofintermolecular disulfide bonds when in the solution; providing atemplate DNA molecule comprising a T7 promoter functionally linked to atarget nucleotide sequence to be transcribed; contacting, within thesolution, the template DNA molecule with the variant in the presence ofribonucleoside triphosphates; and incubating the solution, following thestep of contacting, under conditions favoring RNA polymerase activity,thereby synthesizing the RNA molecule.
 18. The method according to claim15, wherein the variant further comprises a Cysteine-Serine substitutionselected from the group consisting of Cys125Ser, Cys347Ser, Cys492Ser,Cys515Ser, and Cys839Ser.
 19. The method according to claim 15, whereinthe variant further comprises an amino acid substitution selected fromthe group consisting of Val426Leu, Val426Ile, Val426Phe, Ser633Val,Ser633Met, Val650Leu, Thr654Leu, Ala702Val, and Val795Ile.
 20. Themethod according to claim 15, wherein the variant comprises amino acidsubstitutions Val426Leu, Ala702Val, and Val795Ile.